1. Field of the Invention
The present invention relates to a liquid crystal display device, more particularly to an active matrix type liquid crystal display device using a thin film transistor as a switching element.
2. Description of the Prior Art
In an active matrix type liquid crystal display device (hereinafter referred to as an LCD) driven by a thin film transistor (hereinafter, referred to as a TFT), the TFT having a reverse stagger type structure (a bottom gate type structure) has been used in general.
As shown in FIGS. 1A and 1B, the LCD has been heretofore constituted of a gate electrode 2 formed on a transparent insulating substrate 12, a gate wiring 1 connected to the gate electrode 2, a semiconductor thin film 5 formed on the gate electrode 2 via an insulating film 10, drain and source electrodes 4 and 7 formed so as to be connected to the semiconductor thin film 5, a drain wiring 3 formed integrally with the drain electrode 4, which crosses the gate wiring 1 interposing the insulating film 10, and a transparent pixel electrode 8 formed so as to be connected to the source electrode 7.
In this conventional LCD, a lateral direction electric field between the transparent pixel electrode and the drain wiring is produced, because the transparent pixel electrode and the drain electrode are made of the same layer. It should be noted that when a liquid crystal is applied with a voltage between a pixel electrode and an electrode facing the pixel electrode, both being formed on a TFT substrate, a direction from the pixel electrode to the opposing electrode is defined as a longitudinal direction. A stronger lateral direction electric filed is produced when a distance between the pixel electrode and the drain wiring is reduced as the LCD is micronized to a higher degree.
With reference to FIGS. 2 and 3, when a liquid crystal is supplied with a voltage, a liquid crystal normally rises into a pretilt direction 15 which is caused according to a rubbing direction for an orientation film. However, when the aforementioned strong lateral electric field is present in reverse to the pretilt direction, the liquid crystal is oriented into this lateral electric field direction. As a result, in the region where the electric field opposite to the pretilt direction is present, the liquid crystal 13 rises to a reverse direction to a normal direction, that is called a reverse tilt state. When the reverse tilt state is caused, a transition region called a disclination 16 is generated at a boundary between the normal tilt region, called a normal tilt state, and the region of the reverse tilt 17, as shown in FIG. 3. This disclination region (bounded by lines 16 in FIG. 3) transmits light unfortunately when a normally black LCD operates a black display. Hence, is display contrast is deteriorates.
To shield the aforementioned disclination region, and with reference to FIG. 1C, a black matrix (hereinafter referred to as a BM) layer 20 made of a chromium metal layer or other materials for shielding light, which is formed on a substrate 19 facing the TFT, has been heretofore formed. The BM layer 20 has been formed with an overlapping margin of a considerable width over the pixel electrode to shield the disclination regions, taking into consideration the overlapping precision of the facing substrate with the TFT.
For this reason, an opening area ratio of the BM layer to a normal tilt region (an opening ratio) is reduced, thereby undesirably reducing display brightness. To compensate for the reduction of display brightness, electric power consumption of the LCD increases.
Furthermore, since in the presence of the afore-mentioned strong lateral direction electric field, the disclination area penetrates deeply into the inner pixel electrodes, the BM region has to be formed wider to shield the disclination area. Therefore, there has been a problem that the opening ratio decreases still more.